test: migrate 700_e2e rows to corpus fixtures; retire the carrier

136 rows migrated, 0 already owned, 3 wwstage-failing rows held out.

test/wcc/data/r700_is_as_scalar/case.ww

@@ -0,0 +1,19 @@
+//ww:run-exit 49
+// Migrated from 700_e2e row 113.
+package main;
+fn classify(n: i64) (i64 | i32) = {
+  if (n < 0) { return 7: i32; };
+  return n;
+};
+fn main() i32 = {
+  let ok: (i64 | i32) = classify(40);
+  let bad: (i64 | i32) = classify(-1);
+  let s: i32 = 0;
+  if (ok is i64) { s += 1; };
+  if (bad is i32) { s += 1; };
+  if (ok is i32) { s += 100; };
+  if (bad is i64) { s += 100; };
+  let v: i64 = ok as i64;
+  let e: i32 = bad as i32;
+  return (v: i32) + s + e;
+};